Visual acuity is defined as the reciprocal of the ratio between the letter size that can just be recognized by a patient, relative to the size just recognized by a standard eye . If the patient requires letters that are twice as large (or twice as close), the visual acuity is said to be 1/2 ; if the letters need to be 5x larger, visual acuity is 1/5, and so on.
Note that the scales in the tables extend well above the reference standard (1.0, 20/20, the ability to recognize a letter subtending a visual angle 5 min. of arc), since normal acuity is often 1.25 (20/16), 1.6 (20/12.5) or even 2.0 (20/10).
Today, the ETDRS chart and ETDRS protocol, established by the National Eye Institute in the US, are considered to represent the de-facto gold standard for visual acuity measurements The International Council Of Ophthalmology, Visual Standard, Aspects and Ranges of Vision Loss (April, 2002) is a good reference document.
The full ETDRS protocol requires a wide chart, in the shape of an inverted triangle, on a light box, and cannot be implemented on the limited screen of a projector (or similar) chart.
For most routine clinical measurements projector charts or traditional charts with a rectangular shape are used; these non-standardized tools are less accurate than ETDRS measurements.
This appendix contains two lookup tables, one for traditional charts and one for ETDRS measurements.